1. Field of the Invention
The present invention relates to magnetoresistive sensors for reading magnetically-recorded information from data storage media, and particularly to MTJ (Magnetic Tunnel Junction and CPP (Current-Perpendicular-to-Plane) spin valve read sensors for direct access storage device (DASD) systems.
2. Description of the Prior Art
By way of background, MTJ and CPP spin valve devices have been proposed for use as magnetic field sensors (e.g., read head sensors) in DASD systems, such as disk drives. An MTJ sensor is a variable current device formed by a magnetically pinned ferromagnetic layer and a magnetically free ferromagnetic layer separated by an intervening tunnel barrier layer. The barrier layer is made from a material that is electrically insulating but thin enough to pass charge carriers via a quantum mechanical phenomenon known as electron spin tunneling. Electrical leads and/or shields are positioned to make electrical contact with the ferromagnetic layers and induce a sense current that passes perpendicularly through the tunnel barrier layer from the pinned ferromagnetic layer to the free layer. Because the tunneling phenomenon is electron-spin dependent, the electrical resistance across the barrier layer is related to the spin polarity of the tunneling current, which in turn is a function of the alignment of the magnetic moment (magnetization directions) of the free layer relative to that of the pinned layer. The electron tunneling probability is highest when the magnetic moments are parallel and lowest when the magnetic moments are antiparallel. Because the magnetic moment of the free layer can be influenced by an external magnetic field, such as a magnetic domain on a data storage medium, an MTJ sensor's electrical resistance will vary as a function of the direction and magnitude of the recorded magnetic domains, which rotate the free layer's magnetic moment with respect to the relatively fixed magnetic moment of the pinned layer.
A CPP spin valve sensor is generally similar in construction to an MTJ sensor except that the tunnel barrier layer is replaced with an electrically conductive barrier layer. The CPP spin valve sensor thus comprises a stacked array of a pinned ferromagnetic layer, an electrically conductive metallic barrier layer, and a free ferromagnetic layer, sandwiched between a pair of electrical leads and/or shields. Like an MTJ sensor, electrical current in a CPP spin valve sensor is passed perpendicularly to the planes of the stacked array layers, and electrical resistance is measured. The sensor readback signal is a function of the resistance changes that result when the magnetic moment of the free layer rotates relative to the pinned layer magnetic moment under the influence of recorded magnetic domains. Resistance is lower when the relative magnetic moments are parallel and higher when the magnetic moments are antiparallel. Because of their superior sensitivity, CPP spin valves are generally preferred over CIP (Current-In-Plane) spin valves.
Commonly assigned U.S. Pat. No. 5,898,547 of Fontana et al. (the '547 patent), discloses a magnetoresistive read head (with an MTJ sensing element) for use in an integrated read/write head of a disk drive. The sensing element of the read head is recessed from the sensing surface of the head to minimize damage to the sensing element layers during lapping. A flux guide is formed as an extension of the sensing element free layer and carries magnetic flux from the sensing surface to the sensing element.
A disadvantage of the read head of the '547 patent is that the flux guide and the sensing element structures are defined by separate photo processing operations, and the flux guide has a track width which is narrower than that of the sensing element. It is believed that this reduces read sensitivity because the magnetic flux carried from the narrower track width of the flux guide into the wider track width of the sensing element device becomes diluted.
Another disadvantage of the read head of the '547 patent stems from the use of hard biasing material to longitudinally bias the sensing element free layer for noise reduction and improvement of signal-to-noise characteristics. In the disclosed read head, the direction of longitudinal biasing is parallel to the sensing surface and across the track width of the free layer. The hard biasing material is disposed on either side of the track width boundaries of the free layer, but because it is electrically conductive, the biasing material must be spaced from the sensing element free layer and separated therefrom by an insulator. In order to have good insulating quality, the insulator must be relatively thick. Unfortunately, the required insulating thickness is sufficiently large that hard biasing effectiveness relative to the free layer is lost.
It is submitted that improvements in the design of magnetoresistive sensors, particularly recessed sensor devices of the type disclosed in the '547 patent, are needed in order to overcome the aforementioned disadvantages. What is required is an magnetoresistive sensor in which flux delivered to the sensing element is not diluted, and wherein increased longitudinal biasing strength is provided.